1. Field of the Invention
The present invention relates to a zoom lens and an image pickup apparatus equipped with the same.
2. Description of the Related Art
Heretofore, as large-diameter standard zoom lenses, zoom lenses disclosed in Japanese Patent Application Nos. 2007-93976 and 2008-122676 have been known. The zoom lenses disclosed in Japanese Patent Application Nos. 2007-93976 and 2008-122676 are used as interchangeable lenses. The zoom lenses disclosed in Japanese Patent Application Nos. 2007-93976 and 2008-122676 have certain degrees of brightness throughout the entire zoom range.
The zoom lens disclosed in Japanese Patent Application No. 2007-93976 is a five-unit zoom lens, and a refractive power arrangement is, in order from the object side, a negative refractive power, a positive refractive power, a negative refractive power, a positive refractive power, and a positive refractive power. In this zoom lens, during zooming, the first to fourth lens units move, while the fifth lens remains stationary. When a frame that holds the optical system is regarded as a cylinder, the stationary fifth lens unit can function as a lid on one end of the cylinder. Therefore, the fifth lens unit that remains stationary helps reduction of dust entering the optical system. Some camera bodies are equipped with a microphone with which ambient sound can be captured at the time of shooting. In such cases, the stationary fifth lens unit helps reduction of operation noise generated by the zooming operation.
As to focusing, the zoom lens disclosed in Japanese Patent Application No. 2007-93976 employs the internal focusing scheme. Specifically, a sub lens unit in the second lens unit is moved during focusing.
The zoom lens according to Japanese Patent Application No. 2007-93976 having the above-described construction has a focal length that varies from approximately 24 mm to 50 mm by zooming and an F-number approximately equal to 2.8 over the entire zoom range.
The zoom lens disclosed in Japanese Patent Application No. 2008-122676 is a four-unit zoom lens, and a refractive power arrangement is, in order from the object side, a positive refractive power, a negative refractive power, a negative refractive power, and a positive refractive power. In this zoom lens, all the lens units move during zooming.
As to focusing, this zoom lens employs what is called floating mechanism. Specifically, the second lens unit and the third lens unit are moved independently during focusing.
The zoom lens disclosed in Japanese Patent Application No. 2008-122676 having the above-described construction has a focal length that varies from approximately 14 mm to 35 mm by zooming and an F-number approximately equal to 2.0 over the entire zoom range. This focal length range of 14 mm to 35 mm is equivalent to a focus range of 28 mm to 70 mm in the 135 format (which is commonly called Leica format). (In other words, the zoom lens according to the Japanese Patent Application No. 2008-122676 has an angle of view equivalent to that resulting from the focal length variation from 28 mm to 70 mm in the 135 format.)